Yasuharu Sugawara

Redshifted emission lines and radiative recombination continuum from the Wolf-Rayet binary θ Muscae: evidence for a triplet system?

We present XMM-Newton observations of the WC binary Theta Muscae (WR 48), the second brightest Wolf-Rayet binary in optical wavelengths. The system consists of a short-period (19.1375 days) WC5/WC6 + O6/O7V binary and possibly has an additional O supergiant companion (O9.5/B0Iab) which is optically identified at a separation of ~46 mas. Strong emission lines from highly ionized ions of C, O, Ne, Mg, Si, S, Ar, Ca and Fe are detected. The spectra are fitted by a multi-temperature thin-thermal plasma model with an interstellar absorption N_H = 2--3*10**21 cm**-2. Lack of nitrogen line indicates that the abundance of carbon is at least an order of magnitude larger than that of nitrogen. A Doppler shift of ~630 km/s is detected for the OVIII line, while similar shifts are obtained from the other lines. The reddening strongly suggests that the emission lines originated from the wind-wind shock zone, where the average velocity is ~600 km/s. The red-shift motion is inconsistent with a scenario in which the X-rays originate from the wind-wind collision zone in the short-period binary, and would be evidence supporting the widely separated O supergiant as a companion. This may make up the collision zone be lying behind the short-period binary. In addition to the emission lines, we also detected the RRC (radiative recombination continuum) structure from carbon around 0.49 keV. This implies the existence of additional cooler plasma.

Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57) cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10^(34-39) ergs. Notably, the peak X-ray luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau and L_X, respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

Suzaku monitoring of the Wolf-Rayet binary WR 140 around periastron passage: An approach for quantifying the wind parameters

Suzaku observations of the Wolf-Rayet (W-R) binary WR 140 (WC7pd+O5.5fc) were made at four different times around periastron passage in 2009 January. The spectra changed in shape and flux with the phase. As periastron approached, the column density of the low-energy absorption increased, which indicates that the emission from the wind-wind collision plasma was absorbed by the dense W-R wind. The spectra can be mostly fitted with two different components: a warm component with kBT = 0.3-0.6 keV and a dominant hot component with kBT ∼ 3 keV. The emission measure of the dominant, hot component is not inversely proportional to the distance between the two stars. This can be explained by the O star wind colliding before it has reached its terminal velocity, leading to a reduction in its wind momentum flux. At phases closer to periastron, we discovered a cool plasma component in a recombining phase, which is less absorbed. This component may be a relic of the wind-wind collision plasma, which was cooled down by radiation, and may represent a transitional stage in dust formation.

Suzaku Detection of an Intense X-Ray Flare from an A-Type Star, HD161084

We report on a serendipitous detection of an intense X-ray flare from the Tycho reference source on HD 161084 during a Suzaku observation of the galactic center region for � 20 ks. The X-ray Imaging Spectrometer recorded afl are from this A1-type dwarf or subgiant star with a flux of � 1.4 � 10 � 12 erg s � 1 cm � 2 (0.5–10 keV) and a decay time scal eo f� 0.5 hr. The spectrum is hard with a prominent Fe XXV K˛ emission line at 6.7 keV, which is explained by a � 5k eV thin-thermal plasma model attenuated by a � 1.4� 10 21 cm � 2 extinction. The low extinction, which is consistent with the optical reddening, indicates that the source is a foreground star toward the galactic center region. Based on a spectroscopic parallax distance of � 530 pc, the peak X-ray luminosity amounts to � 1 � 10 32 erg s � 1 (0.5–10 keV). This is much larger than the X-ray luminosity of ordinary late-type main-sequence stars, and the X-ray emission is unattributable to a hidden late-type companion that comprises a wide binary system with the A star. We discuss possible nature of HD 161084, and suggest that it is most likely an interacting binary with elevated magnetic activity in the companion, such as the Algol-type system. The flux detected by Suzaku during the burst is � 100times larger than the quiescent level measured using the archived XMM-Newton and Chandra data. The large flux

A Universal Correlation between the Duration and the X-ray Luminosity in Stellar Flares

Since the launch in 2009 August, with the unprecedentedly high sensitivity as an all-sky X-ray monitor, MAXI has caught more than a hundred of huge flares from stars. Most of them are from low-mass, active stars (RS CVn systems, an Algol system, dMe systems, a dKe system, Young Stellar Objects). With the total radiative energy of 1034–1039 ergs, the MAXI detections have broken the record of the largest flaring magnitudes in each stellar categories. The enlarged sample of intense flares has enabled us to do systematic studies in various viewpoints. One of the studies is the discovery of a universal correlation between the flare duration and the intrinsic Xray luminosity, which holds for 5 and 12 orders of magnitude in the duration and LX, respectively (Tsuboi et al. 2016). Here, we review the studies of stellar flares obtained with MAXI.

An X-ray study of mass-loss rate and wind acceleration of massive stars

By monitoring WC7 and the O5.5 binary WR 140 with the Suzaku telescope, we demonstrate a new method to measure the mass loss rates of both stars. By using the absorption column density, we found a mass-loss rate for the WC7 component : ṀWC7 ≈ 1.2×10−5M yr−1 . We also measured the mass-loss rate of the companion O component using a luminosity variation in phases: ṀO5 .5 ≈ 5 × 10−7M yr−1 .

Suzaku monitoring of the Wolf‐Rayet binary WR140

We report the preliminary results of the Suzaku observations of the W‐R binary WR 140 (WC7+O5I). We executed the observations at four different epochs around periastron passage in Jan. 2009 to understand the W‐R stellar wind as well as the wind‐wind collision shocks. The total exposure was 210 ksec. We detected hard X‐ray excess in the HXD band (>10 keV) for the first time from a W‐R binary. Another notable discovery was a soft component which is not absorbed even by the dense wind. The spectra can be fitted by three different components; one is for the stationary cool component with kT ∼0.1 keV, one for a dominant high temperature component with kT ∼3 keV, and one for the hardest power‐low component with Γ∼2. The column density at periastron is 30 times higher than that at pre‐periastron, which can be explained as self‐absorption by the W‐R wind. The emission measure of the dominant, high temperature component is not inversely proportional to the distance between the two stars.We report the preliminary results of the Suzaku observations of the W‐R binary WR 140 (WC7+O5I). We executed the observations at four different epochs around periastron passage in Jan. 2009 to understand the W‐R stellar wind as well as the wind‐wind collision shocks. The total exposure was 210 ksec. We detected hard X‐ray excess in the HXD band (>10 keV) for the first time from a W‐R binary. Another notable discovery was a soft component which is not absorbed even by the dense wind. The spectra can be fitted by three different components; one is for the stationary cool component with kT ∼0.1 keV, one for a dominant high temperature component with kT ∼3 keV, and one for the hardest power‐low component with Γ∼2. The column density at periastron is 30 times higher than that at pre‐periastron, which can be explained as self‐absorption by the W‐R wind. The emission measure of the dominant, high temperature component is not inversely proportional to the distance between the two stars.

An X‐ray Study of a Massive Star and its Wind

WR 140 is one of the best known examples of a Wolf‐Rayet stars. We executed the Suzaku X‐ray observations at four different epochs around periastron passage in Jan. 2009 to understand the W‐R stellar wind as well as the wind‐wind collision shocks. The column density at periastron is about 30 times higher than that at pre‐periastron, which can be explained as self‐absorption by the Wolf‐Rayet wind. The spectra are dominated by a line and continuum emission from a optically thin‐thermal plasma. The strong Ne‐K lines are evidence that the thermal plasma is shock‐heated W‐R wind materials by the interaction with the wind from the companion O star. We present the parameters of the wind, such as a mass‐loss rate, which were calculated with the absorption and line emission in the spectra.